Many telephone station sets and telephone networks are designed to transmit voice signal spectral components at frequencies greater than about 300 Hz with greater efficiency than spectral components below about 300 Hz. The resulting attenuation of spectral components below about 300 Hz can degrade the perceived quality of voice signals transmitted from such telephone sets over telephone networks to other telephone sets.
U.S. Pat. No. 5,195,132 discloses methods for preferentially amplifying voice signal spectral components between about 100 Hz and about 300 Hz in a telephone network in an effort to compensate for the low efficiency of many telephone transmitters in this frequency range. The preferential amplification is implemented by suitable programming of digital signal processors of echo cancellation circuitry provided at toll offices of the telecommunications network. (U.S. Pat. No. 5,195,132 issued Mar. 16, 1993 in the names of D. O. Bowker et al, and is entitled "Telephone Network Speech Signal Enhancement". The entire specification of this patent is hereby incorporated by reference.)
Unfortunately, the indiscriminate application of such preferential amplification to all signals regardless of their source may degrade the quality of some signals rather than enhancing it. For example, voice signals transmitted by telephone sets having relatively high efficiency for spectral components below 300 Hz will be overcompensated with a resulting degradation in signal quality.
U.S. Pat. No. 4,847,885 discloses a system for measuring and automatically compensating for distortions caused by a connection between a telephone station set and a central voice processing unit, such as a voice-mail system. The system comprises a sound source for producing reference sounds, an acoustic coupler for coupling the reference sounds to a telephone set for transmission to the central voice processing unit via a communications path, a sound analyzer coupled to the voice processing unit for spectrum analyzing sounds received at the central voice processing unit, a memory storing spectra of the reference sounds, and a processor for comparing the spectra of the received sounds to corresponding spectra of reference sounds. The comparison of received spectra to reference spectra permits computation of distortions caused by the telephone set and communications path. The processor then automatically compensates for distortion caused by the telephone set and communications path in processing speech received over the telephone connection. (U.S. Pat. No. 4,847,885 issued Jul. 11, 1989 in the name of V. Vittorelli, and is entitled "System for Measuring and Automatically Compensating for the Distortions of a Connection Between a Telephone Apparatus and a Central Voice Processing Unit".)
Unfortunately, the system disclosed in U.S. Pat. No. 4,847,885 requires portable units for acoustically coupling reference sounds into telephone sets. Compensation is available only to callers having such portable units, and those callers must take the time to use the units to couple reference sounds into the telephone sets at the beginning of each call. The callers must take care to properly position the portable units relative to the telephone set microphones to get good results. Consequently, the system disclosed in U.S. Pat. No. 4,847,885 is relatively expensive to implement and cumbersome for callers to use.